mpegenc: use avctx->slices as number of slices

[ffmpeg] / tests / ref / fate / h264-conformance-frext-frext3_panasonic_d

0, 0, 152064, 0xdd346292
0, 3600, 152064, 0xb07e6efd
0, 7200, 152064, 0x81b4d540
0, 10800, 152064, 0xc50066cf
0, 14400, 152064, 0x30a49588
0, 18000, 152064, 0x57520bfa
0, 21600, 152064, 0xc0409dd9
0, 25200, 152064, 0x8e6453e9
0, 28800, 152064, 0x7ca86d86
0, 32400, 152064, 0xf937b3b5
0, 36000, 152064, 0x6c1f4604